1. Field of the Invention
The present invention relates generally to a multilayer integrated-circuit solar module in which a plurality of stacks of layers of thin-layer solar cells are disposed alongside one another on a substrate.
2. Description of the Related Art
On exposure to light, solar cells and solar modules comprising amorphous silicon (a-Si:H) exhibit a substantial deterioration in the electrical power, which may amount to 40 percent of the original power. Although this aging process which occurs during the operation of the solar cells can be reversed by heat treatment at 180.degree. C., it starts again on exposure to light.
One way of avoiding this aging effect, which is also referred to as the Staebler-Wronski effect in solar cells, is to use mixed semiconductors having a smaller band gap to produce the active i-type layer. It is also possible to produce p-i-n solar cells comprising amorphous silicon with thinner i-type layers which exhibit hardly any aging below a layer thickness of approximately 300 nm. However, such a thin i-type layer only incompletely absorbs the red component of the solar spectrum with the result that an unduly low short-circuit current is obtained with these cells.
One way of producing solar cells having thinner i-type layers but high red absorption is offered by stacked cells. Such a stacked cell made up of two or more individual cells which are each thin on their own absorbs an adequate portion of the sunlight. A double cell has the structure p-i-n/p-i-n, a triple cell has the structure p-i-n/p-i-n/p-i-n, and so on. The i-type layers of the subcells of a stack may comprise the same semiconductor material or be produced from various semiconductor materials, for example from silicon/carbon or silicon/germanium alloys.
Tandem cells or, generally, stacked cells, have in fact a higher resistance to light aging, with a constant or even a higher efficiency. In such cases, the cells in a stack are optically arranged one behind the other and, in the simplest case, are electrically connected in series. The photovoltages of cells situated one above the other are added together, while the photocurrents must be identical. The maximum achievable photocurrent in such a stacked cell is determined by the photocurrent of the weakest individual cell.
The current adjustment or, alternatively, the "current matchings" can, however, be carried out only with difficulty, in particular in the production of large-area solar modules. Layer thickness variations in the deposition of the active semiconductor layer occur to a greater extent with increasing substrate area and affect the photocurrent.
In order to avoid the problem of current matching, European Patent Application 0 326 857 and corresponding U.S. Pat. No. 4,948,436 proposed for the first time to use stacked cells having a p-i-n-i-p or n-i-p-i-n structure and to interconnect them in an integrated manner in a module. In this arrangement, the two solar cells, situated one above the other, of the tandem cell are connected electrically in parallel, in which case the photocurrents of the subcells are added together. Only a voltage adjustment of the subcells is necessary in order to have photovoltages which are as identical as possible. Since, however, there is only a slight dependence of the photovoltage on the photocurrent in the working range of a solar cell, the voltage matching of a tandem cell or stacked cell having a parallel interconnection can be carried out markedly more easily than a current matching in the case of a series connection.
A disadvantage of the known tandem solar module having a parallel interconnection is the structure, which is complicated compared with a simple thin-layer solar module, which requires an edge insulation in the isolating trenches, or grooves, between the stacked cells and in which contact is made via steps in the layers for the purpose of interconnection.